1. Technical Field
This invention relates generally to earth boring bits and in particular to the pattern of placement of cutting elements on a rolling cutter.
2. Background Information
Various forms of earth boring bits are utilized to cut through the hard material formations in the earth when forming a well bore. One general form of drill bit utilizes one or more rolling cutters whose outer surfaces include projections such as milled teeth or cutter inserts that gouge into the formation material causing the material to disintegrate or pulverize as the cutter is rotated when the tool is turned about its axis. The rolling cutters are individually mounted to rotate about a supporting shaft or spindle typically with the axis of the spindle spaced radially from and at an incline with respect to the rotational axis of the tool. The incline of the spindle axis causes the cutter to both rotate about its axis and roll relative to the bottom of a borehole as the bit body is rotated. As a result, the cutter disintegrates a concentric ring of formation material in the bottom of the borehole.
One earlier version of the foregoing general type of rolling cutter is disclosed in U.S. Pat. No. 3,389,760. The patent discloses a rolling cone cutter supported to rotate upon a load pin which is connected at its opposite ends to a generally U-shaped support saddle. As disclosed, a number of such saddle and rolling cutter arrangements may be mounted on a single bit body for drilling a large borehole. For disintegrating formation, a multiplicity of small inserts of cemented tungsten carbide are fitted into drilled holes in each cutter body. The inserts are disposed in overlapping rows so that as the cutter is rolled over the bottom of a hole the inserts cut overlapping tracks so as to disintegrate the formation over the full width of a concentric swath defined by the length of the cutter as it is rotated around the axis of the drill bit. The cutting elements of U.S. Pat. No. 3,389,760 are in somewhat of a semi-random pattern on a smooth outer surface of the cutter. This physical arrangement of cutting elements leaves certain lateral discontinuities in the bottom hole pattern. As a result, the non-uniform succession of cutting elements often imparts an abrupt impact force during rotation of the cutter. Moreover, by design the outer surface of the cutter does not have relief grooves which initially aid in carrying away a disintegrated formation with the drilling fluid.
Another form of prior art roller cutter is disclosed in U.S. Pat. No. 4,393,949 and includes a helix cutting tooth protruding from the cutter body. The helix shape of the tooth also functions to cut along the full width of the concentric swath relative to the central axis of the bit as the latter is rotated. However, the helix does not close upon itself and as a result, the open-ended helical cutting structure produces a bottom hole pattern that takes the appearance of a series of skewed or spiral open-ended grooves. Because of the open-ended helical design, the lead edge of the cutting tooth is subjected to a potentially abrupt load with each revolution of the cutter about its spindle axis.